gas if ication modelling

8/13/2019 Gas if Ication Modelling

http://slidepdf.com/reader/full/gas-if-ication-modelling 1/2

1st ERCOFTAC Conference onSimulation of Multiphase Flows in Gasification and Combustion,18–21 September 2011, Dresden, Germany

MODELLING OF BIOMASS PYROLYSIS AND GASIFICATION IN

INDUSTRIAL-SCALE GASIFIER

Kamil Kwiatkowski12, Jakub Korotko3, Pawel  Zuk1, Konrad Bajer12

1Faculty of Physics, University of Warsaw 2

Interdisciplinary Centre for Mathematical and Computational Modelling, University of Warsaw 3Faculty of Power and Aeronautical Engineering, Warsaw University of Technology email:   kamil@igf.fuw.edu.pl, korotkoj@gmail.com, pzuk@ippt.gov.pl, konrad.bajer@fuw.edu.pl

and info@biomassgasification.eu

The solution to the problem of dealing with waste, both biomass and municipal, is its utilisation via thegasification process. The direct product of biomass gasification, the biomass syngas, is considered as an attrac-tive, versatile and fully renewable source of energy. It contains mainly nitrogen, hydrogen and carbon monoxideand residual tars and other pollutant.

Almost unlimited diversity in shapes and properties of wastes, complex homo- and heterogeneous chem-istry and complex multiphase flows makes gasification a difficult phenomenon for mathematical and numericalmodelling.

There are two main approaches to pyrolysis and gasification modelling, the first focuses on the detail analysis

of the flow, reactions and heat transfer occurring in one representative biomass particle, the second focuses onmodelling the whole gasification chamber but use several simplifications and subscale models. Driven by theimmediate demand from industry we have been developing the latter approach, focusing on the microscopicphenomena mainly to parametrise them.

In this paper we present the numerical model and its results for industrial-scale biomass gasifier obtained par-allel from our ANSYS Fluent  biomass gasification  UDF-extention (more details in [1])and in-house OpenFOAMporousBiomassGasificationFoam   solver (see also [2]). The model takes into consideration air and syngas flowand reactions within porous layers, water evaporation from biomass, pyrolysis, gasification and heterogeneouscombustion of carbon remnants [3].

Figure 1: Installationof wood chips gasifica-tion and sawdut drying topellets production, Szepi-etowo.

Figure 2: Dry and wetwood chips waiting tobe gasified. The com-position of the woodchips is complex, containsfragmented plywood,chipboard, straw, boards,

bark, sawdust and woodshaving.

The biomass bed formed in the gasification chamber is treated as a porous zone,where Darcy’s law is applicable. This approximation is sufficient, since the Reynoldsnumber is less than 300. Still both Fluent and OpenFoam solve full Navier-Stokes

equation with the additional momentum source term. All parameters of the porouslayer are anisotropic, non-uniform and transient. Generally porosity is increasedin the pyrolysis zone then the bed rapidly crashes and porosity is decreased. Toinitialise the model we made observations in the real-life installation and developedsupplementary zonal and one-dimensional model of gasifier in Matlab (more detailsin   [4]). In the zonal approach each individual zone is artificially separated andmodelled, while in the 1D and CFD approaches there are no artificial division forzones, only the sequence of the process is clarify.

In order to model the four mentioned above processes it is crucial to properlymodel the heat transfer b etween solid (biomass) and gas (air and syngas). Weassumed that the heat exchange in the porous medium is purely convective and wethe used well established heat transfer parameters. Then we introduce the globalheat transfer coefficient based on experimental results.

In order to make our approach versatile, and to deal with various kind of wasteand biomass, we implemented the data structures containing the elemental com-position of solid, its moisture content, volatile particles, fixed carbon and ash con-tent [5]. Additionally volatiles are quantified by fractional composition of lignin,cellulose and hemicellulose for vegetable biomass [6].

We include the homogeneous chemical reactions of water-gas shift, combustionof CO, CH4 ,H2 and heterogeneous reactions such as gasification and char combus-tion. The reaction rates are defined by the chemical kinetics based on the Arrheniusequation (for homogeneous reactions) and the Langmuir-Hinshelwood (for hetero-geneous ones). In the near future we intend to replace the coefficients taken fromliterature [5] by the data obtained from the thermogravimetric measurements thathave already been scheduled.

The results, which we present below are prepared for the industrial updraft,fixed-bed gasifier fuelled by wood chips. It is located in Szepietowo and achieves3.5MW of heat power used for sawdust drying (fig.   1 and 2). The sample resultsobtained from both OpenFOAM and Fluent are presented in figures 3  and  4.  The shape of the feed, marked inthe figures in red, was assumed drawing from the experience of the installation operator. This shape can varydepending mainly on the sizes of the chips. Our model will be experimentally verified for this installation.

1

8/13/2019 Gas if Ication Modelling

http://slidepdf.com/reader/full/gas-if-ication-modelling 2/2

(a) OpenFOAM (b) ANSYS Fluent

Figure 3: Results of our simulations of the flow in a porous layer of biomass (the shape of the porous zone shownin by red) in the simplified biomass gasifier geometry. The velocity fields simulated by the in-house OpenFoamsolver are compared with the result from Fluent (with additional UDF). The air inlets are symmetrically locatedat the bottom. Outlets are located in the top, right-hand side of the gasifier.

Figure 4: Comparison of the pressure distributions obtained with OpenFoam and Fluent. Small disagreementis visible in the lower part of the gasifier. The decrease of pressure with height is in good agreement for bothcodes.

References

[1] Korotko, J., Kwiatkowski, K., Bajer, K. Numerical modelling of thermodynamic processes in biomass bed,IX Workshop Modelling multiphase flows in thermochemical systems, Wiezyca, Poland 2011.

[2] Kwiatkowski,K.,  Zuk, P., Wedolowski, K., Bajer, K. Flow, reactions and production of syngas in the porousbiomass layer. 6th OpenFOAM Workshop, PennState University, USA, 2011.

[3] Souza-Santoz de,M.L.,  Solid Fuels Combustion and Gasification , MARCEL DEKKER Inc., 2004.

[4] Gorecki, B., Kwiatkowski, K., Bajer, K. Numerical simulation of biomass gasification - computational codeand modelling, IX Workshop Modelling multiphase flows in thermochemical systems, Wiezyca, Poland 2011.

[5] Basu, P., Biomass gasification and pyrolysis, Practical design , Elsevier, 2010.

[6] Di Blasi, C., Modelling chemical and physical processes of wood and biomass pyrolysis, Progress in Energyand Combustion Science, 34, 47–90, 2008.

2